Method of alkaline digestion of cellulosic materials



1967 N. HARTLER ETAL 3,

METHOD OF ALKALINE DIGESTION OF CELLULOSIC MATERIALS Filed April 22, 1964 Steamed chi 5 Cooking liquid optionally with addition of carbohydrate preserving chemlouls Heating zone Rel-adsorption promoting ggfi sleps Chemicals Sor ion m Black liquor Washing zone Weak liquor Pulp INVENTORS Nils H6!- P716)- nnie. 4M re 21 izmwm wa/ui ATTO RN EY$ United States Patent 3,354,029 METHOD OF ALKALINE DIGESTION 0F CELLULOSIC MATERIALS Nils Hartler and Ronnie Aurel], Stockholm, Sweden, assignors to Stiftelsen Svensk Cellulosaforskning, Stockholm, Sweden, a corporation of Sweden Filed Apr. 22, 1964, Ser. No. 361,759 Claims priority, application Sweden, May 2, 1963, 4,846/63; Mar. 17, 1964, 3,307/64 Claims. (Cl. 162-41) ABSTRACT OF THE DISCLOSURE The yield of pulp in the chemical digestion of ligneous materials is increased by modifying the operation to reduce degradation of hemicellulose and other polymeric carbohydrates in the cooking liquor and to increase the adsorption of such polymeric carbohydrates on the fiber material during the final stage of the cooking period. This is accomplished in either of two ways: j

(1) withdrawing 50-75% of the cooking liquor at an early stage of the cooking period and contacting the pulp with this liquor during the final stage of the cooking period; and

(2) reducing the pH of the cooking liquor in the final stage of the cooking period by adding sulfuric acid there- If desired, either procedure may be used in combina tion with other known sorption promoting steps or they may be used together. I

simultaneous loss of a relatively great portion of the carbohydrates of the wood, in particular hemicelluloses. This results, in addition to the loss of the lignin, in a yield reduction corresponding to about 25 to 30% of the weight of the wood for chemical pulps. A major portion of this loss is caused by a progressive degradation, so-called peeling, which starts from the aldehydic terminal groups of the carbohydrates, whereby acidic low molecular weight degradation products are formed which are immediately dissolved. In addition, polymeric carbohydrate molecules, in particular xylan, are dissolved in the cooking operation. p

Under normal cooking conditions, the maximum amount of non-degraded carbohydrates in the cooking liquid occurs at about 150 C. to 160 C., i.e. just before the maximum temperature in the cook is reached (F. Saarnio and C. Gustafsson, Papper och Tra, 35 (1953) 65; S. Axelsson, I. Croon and B. Enstrdm, Svensk Papperstidning 65 (1962) 693). The amount decreases in the continued cook, on account of degradation, but also on account of the hemicelluloses being partly readsorbed onto the fiber material (S. Yllner and B. Enstrtim, Svensk Papperstidning 59 (1956) 229; 60 (1957) 549) but a certain amount is lost with the cooking liquid after the cook.

The object of the present invention is to increase the yield of pulp by means of special measures aimed at reducing the degradation of solubilized hemicelluloses in the cooking liquor and to regain these by readsorption on the fiber material during the final stage of the cooking period. In order to create favorable conditions. for an increased readsorption in the final stage of the cooking operation special measures are proposed which are included in the invention.

The present invention also contemplates combining the measures referred to above with other measures aimed at stabilizing the solubilized carbohydrates against degradation, whereby a further increased yield of pulp is obtainable.

In one aspect of the present invention, the degradation of solubilized hemicelluloses is reduced by drawing off a major portion (e.g. 50 to by volume) of the cooking liquor at a time in the cooking period, when the content of polymeric carbohydrates in the cooking liquor is still substantial, and preferably when it is at maximum, and returning the drawn off liquor to the digester in the final stage of the cook. This will be referred to hereinafter as liquor transfer and includes returning the drawn off liquor to the same digester, from which it was taken, or introducing it into a separate digester, from which a corresponding amount of cooking liquor had been drawn off at an earlier stage as well as in continuous cooking Withdrawing liquor from a zone, wherein the cellulosic material is in :an initial cooking stage and introducing it into the same apparatus in a zone, wherein the material is in the final cooking stage. During the cooking period a different liquor can be used or the cook can be carried out in the vapor phase. By returning the drawn-off cooking liquor to the digester at the end of the cooking period, a considerably increased yield can be obtained. The amount of dissolved hemicelluloses in the cooking liquor normally increases to about 2 to 4%, based on the wood, in exceptional cases somewhat higher, say 5 or 6%. The withdrawal of liquor should occur when the amount of dissolved hemicellulose is.above 0.l% of the wood, preferably about 1% of the wood and most preferably when it is at a maximum.

In another'aspect of the present invention, the readsorbed quantity and thereby also the yield of pulp can be increased by a step which creates conditions for an increased readsorption in the final stage of the cook. The step which is contemplated is a reduction of the pH of the cooking liquor (by adding e.g. sulfuric acid) toan extent adapted to avoid an undesired precipitation of lignin, which means in general a decrease of pH to within the range of 13 to 11.5.

The pH reduction is suitably achieved by adding sul 'furic acid, in which case the sulfur will be utilized in the .process. The acid may be a low grade acid and e.g. spent acid from purification processes in organic chemical industrial installations or spent coagulating baths from rayon manufacturevcan advantageously be used, in which case any heat content thereof can be utilized, when the cooking liquor is burnt.

In another aspect of the invention, the two abovementioned measures, i.'e, reducing the degradation of hernicelluloses by the aid of'liquor transfer, and creating more favorable conditions for readsorption by reducing the pH of the cooking liquor in the final stage of the cook can be employed together to achieve an even more increased yield of pulp. Other aspects ofthe invention comprise combining either of the two above-mentioned means or the combination of them with further measures, which result in an additionally increased yield. These further measures are directed to increase the stability of the dissolved carbohydrates to degradation by peeling, by eliminating the reducing end groups, hereinafter called carbohydrate preserving steps, and to increase further the readsorption of the hemicelluloses on the fiber material by other sorption promoting steps than that of reducing the pH, such steps being known per se in the art. 1

It is knownthat the yield of carbohydrates and hence the pulp yield can be increased by stabilizing the carbohydrates by reduction with sodium borohydride (N. Hartler, Svensk Papperstidning, 62 (1957), 467), or by oxidation with polysulfides (G. Annergren, Meddelande fran 'CCL, Ser. B. No. 46, June 1961, page 42 When using these methods, the dissolved carbohydrates are also stabilized in the cooking liquor, and it has been established that this occurs most markedly if the reduction or oxidation is carried out during the entire cooking period, whereby also the aldehydic groups formed during the cook are stabilized. Thus, by this method it is possible to increase the proportion of readsorbable carbohydrates in the cooking liquor.

This effect is achieved independent of that of the liquor transfer or pH reduction and if the carbohydrate preserving step is combined with them an additive effect is achieved as the net result.

The contemplated modification of the terminal groups of the carbohydrates can be achieved by means of reducing agents, other than borohydrides, which have a reducing action on aldehyde group s, e.g. various aminoboranes, hydroxylamine or hydrazine. Since a reduction of the terminal groups of the dissolved carbohydrates is primarily intended, a catalytic hydrogenation with gaseous hydrogen on heterogeneous catalysts, such as nickel, rheninm, ruthenium or ruthenium dioxide, is also possible. Homogeneous catalysts are also possible in hydrogenation with gaseous hydrogen, e.g. hydridopentacyano cobaltate. t t n I n A sorption promoting step which can be used in accordance with this invention is a temperature reduction of the cooking liquor in the final stage of the cook or of the transferred liquor in the case of the liquor transfer methd. The phenomenon occuring therein is probably analogous to the formation of crystallization nuclei. By the presence of such crystallization nuclei, association between different molecules inthe solution is facilitated. It is probable that the increased extent of, the readsorption by temperature reduction can be explained in a similar manner. A substantial temperature reduction is to be recommended, but evena reduction of 30 to 40 is eife'ctive inthis connection.

A temperature reduction followed a temperature increase is a further sor'p tion promoting step. It is known that the extent of hemicellulose adsorption is increased with increasing temperature (N. Hartler and A. Lund, Svensk Papperstidning 65 (1962) 951), wherefore a 'icombinatio' n of a temperature decrease with the above described effect as a result and a subsequent temperature increase gives a somewhat increased sorption effect as compared to temperature re'duction alone. The reason why a temperature increase is favorable is probably related to the fact that hernicelluloses in aqueous solutions are hydrated. :Since the hydration is unfavorable from the point of view of sorption, it is to be expected that a temperature. increase increases the sorption, as the hydration of the carbohydrates 'is reduced. 7

n It is also known that if the cooking liquor and the wood material are cooled together, a further readsorption is achieved. This method is, of course, also applicable in connection with this invention.

Thecarbohydrate preserving steps and the sorption promoting steps function independently of each other and can be combined. A number of experimentally tested combinations appear in the examples below.

The liquor transfer procedure is particularly suitable for continuous digestion in the manner shown more-orless diagrammatically in the accompanying drawing. As indicated by the legends and arrows in the single figure, wood chips and conventional alkaline cooking liquor are supplied to the heating section of a continuous digestion zone wherein the mixed feed is progressively heated to the digestion temperature, usually about 170 C., for soda cook. Before the maximum temperature is reached, from 50 to 75% of the cooking liquor is withdrawn and, bypassing the digestion zone, is readmitted to the system in the-sorption zone.

'After leaving the sorption zone, spent, or black, liquor EXAMPLE 1 Sulfate cooking of commercial birch chips was carried out in acid-resistant autoclave tubes containing 300 g. of chips (dry basis) at a liquid-to-wood ratio of 4 and with a charging of sodium hydroxide and sodium sulfide in such quantities that the alkali ratio, calculated as effective alkali, was 22.5% and the sulfidity Charging took place at 70 C. The temperature was increased linearly from 70 C. to 170 C. in 2 hours. The cooking time at 170 C. was one hour. Thereafter, 200 ml. of 0.6 M sulfuric acid were injected into the autoclave, whereupon cooking was continued for 15 minutes more.

The pulp was washed, defibrated and screened, whereupon yield, kappa number and brightness were determined.

EXAMPLE 2 Sulfate cooking of commercial birch chips was carried out as described in Example 1 but with. 18.5% 'efiective alkali and with an addition of 2% of sodium borohydride (NaB I-L based on the wood, to the cooking liquor. Addition of acid and working up were as described in Example l, the brightness of the pulp was not determined, however.

Results:

With Without injection injection of acid of acid pH in cooking liquor. '12. 5 13. 1 Screened yield, percent:

Average 57.2 55.5 Range of variation 0. 8 l. 1 Kappa. number 16. 3 15. 7

EXAMPLE 3 Sulfate cooking of commercial birch chips was carried out in a digester equipped with means for liquid circulation and indirect heating with steam. 1000 g. of chips (dry basis) were charged, the liquid to wood ratio was 4:1 and the chemical charge was 20% effective alkali and 25% sulfidity. The cook was started at 70 C. and the temperature was raised linearly to the cooking temperature, C., in 2 hours. The cooking timeat 170 C. was one hour, whereupon 600 ml.of 0.6 M H 50 were injected and the cook continued for additionally 15 minutes. In the cook, the cooking liquid was passed through a catalyst bed of ruthenium dioxide disposed 'in the circulation conduit, through which bed hydrogen was blown.

Results:

With Without injection injection of acid of acid pH in cooking liquor 12. 4 13.0 Screened yield, percent 52. 7 50. 9 Kappa number 18 6 18.5

EXAMPLE 4 Sulfate cooking of birch chips was carried out as described in Example 1, but with 18.5% effective alkali and an addition of 0.5% sodium borohydride, based on the wood, to the cooking liquor. During the cook, 100 ml./ h. of a solution containing 10 g./l. of NaBI-L, and 4 g./l. of NaOH was injected. The injection of the NaBH; solution was interrupted at the addition of acid, but was continued thereafter.

Results:

With Without injection injection of acid of acid pH in cooking liquid 12. 6 13. 2 Screened yield, percent.. 57. 3 v54. 8 Kappa number- I 15. 7 15. 1

EXAMPLE Sulfate cooking of birch chips was carried out as in Example 1, except that the cooking liquor was drawn oif through a cooling coil minutes before the maximum temperature, 170 C., was reached. The cooking liquor withdrawn was reinjected into the digester after the cooking period, whereupon the temperature was maintained at 160C. for 20'minutes. In a parallel cook 200 ml. of 0.6

M H 80 were also injected after the cooking liquor.

Results:

Sulfate cooking of commercial birch chips was carried out in autoclave tubes containing 300 g. of chips (drybasis) at a liquid to wood ratio of 4 and with NaOH and Na S charged in such amounts that the alkali ratio, calculated as efiective alkali, was 20% and the sulfidity 25% The cooking temperature was raised linearly from 70 C. to 170 C. in 100 minutes. 10 minutes before the maximum temperature was reached, 600 m1. of cooking liquor were withdrawn. The withdrawn cooking liquid was immediately injected into a parallel cook, which was started 45 minutes earlier and from which 600 ml. of cooking liquor had been withdrawn 10 minutes before the maximum temperature was reached, whereupon this cook was continued for 10 minutes more at 170 C. As a comparison, a cook was carried out without liquor transfer with the same chemical charging and \with a cooking time of 45 minutes at 170 C. The pulps were washed, defibrated and screened, whereupon the kappa number and the yield were determined.

Normal Liquor cook transfer pH in black liquor 12. 9 12.8 Screened yield, percent. 51.4 52. 4 Kappa number 17.9 17.9

EXAMPLE 7 Sulfate cooking of commercial birch chips, including liquor transfer, was carried out as in Example 6, except that the parallel cook was started 55 minutes earlier. Immediately after the transfer of the withdrawn liquor, 200 ml. of 0.5 M H were injected.

Normal Liquor cook acc. transfer to Ex. 6 and acid injectlon pH in black liquor 12.9 12.2 Screened yield, percent-.- 51. 4 53. 0 Kappa number 17. 9 18. 2

EXAMPLE 8 Sulfate cooking of commercial birch chips was carried out as in Example 6, the liquor being withdrawn through a cooling coil and transferred in the same manner as in Example 1. In a parallel run carried out in accordance with'Example 7, 200 ml. of 0.5 M H 80 were injected immediately after addition of the withdrawn liquor.

Results: j

' Liquor Liquor Normal transfer transfer, cook ace. and cooling to Ex. 6 cooling and acid to 0. injection pH in black liquor 12.9 12.7 -12. 2 Screened yield, percent. 51. 4 52. 9 53. 4 Kappa number 17. 9 18. 1 18. 9

EXAMPLE 9 Sulfate cooking of commercial birch chips was carried out according to Example 7, the cooking liquor being withdrawn through a cooling coil and transferred to the parallel cook. The temperature was then lowered to C. at which temperature the cook was continued for 15 minutes. A parallel run was carried out in the same manner, wherein 200 ml. of 0.5 M H 50 were injected Sulfate cooking of commercial pine chips was carried out in autoclaves containing 300 g. of chips (dry basis) at a liquid to wood ratio of 4 and at 18.5 effective alkali and 25% sulfidity. To the cook was also added 0.5% of NaBH based on the wood. The temperature was raised linearly from 70 C. to C. in 100 minutes. 600 ml. of cooking liquor were withdrawn (a) 10 minutes before the maximum temperature was reached, (b) as the maximum temperature was reached and (c) 15 minutes after the maximum temperature was reached. The cooking liquor was directly transferred to a parallel cook started so much earlier that 90 minutes of cooking time at 170 C. was maintained in each case and from which 600 ml.

of cooking liquor had been drawn off at corresponding times. The cook was then continued for 15 minutes at 170 C. As a control, a cook was carried out without liquor transfer, with the same charging as above and with Sulfate cooking of commercial pine chips was carried out in autoclaves containing 300 g. of chips (dry basis) at a liquid to wood ratio of 4 and with 18.5% effective alkali and 25% sulfidity. In the cooking liquor, 3% of sulfur was dissolved based on the wood. The temperature was raised linearly from 70 C. to 170 C. in 100 minutes. When the maximum temperature was reached, 600 ml. of cooking liquor was drawn off. The cooking liquor was directly transferred to a parallel cook started 90 minutes earlier. From this cook, 600ml. of cooking liquor had been withdrawn when the cooking temperature was reached and at'the same time 200 ml. of M NaOH had been injected. The cook was continued for 15 minutes after the liquor transfer "at 170 C. A similar cook with injection of 100 ml. of 0.5 M H 80 after the liquortransfer was also carried out, but in this case the parallel cook had been started a further 15 minutes earlier. A control cook was carriedout with the same chemical charging, but with no special measures.

Results:

Normal Liquor cock with Liquor transfer, 3% sulfur transfer acid injection pH in black liquor 12. 4 12. 7 12. 5 Screened yield, percent. 51. 0 52.9 53. 4 Kappa number 40. 7 37. 8 39. 1

We claim:

1. In the preparation of cellulose pulp wherein cellulosic materials are digested with an alkaline cooking liquor at an elevated temperature in a cooking period, the improvement which comprises drawing oif from 50 to 75% by volume of the cooking liquor during an early stage of the cooking period when the proportion of hemicelluloses therein is in the range 0.1 to 6% based upon the cellulosic material being digested, and admixing said withdrawn liquor with cellulose pulp and cooking liquor during the final stage of the cooking period whereby hemicelluloses in said withdrawn liquor are absorbed on the pulp without substantial degradation of said hemicelluloses.

2. A method as in claim 1, in which the temperature of the drawn off liquor is reduced at least 30-40 C.

3. A method as in claim 1, in which there is a reduction of at least 30-40 C., in the temperature of the liquor and of the cellulosic material after the admixing.

4. A method as in claim 1, which comprises adding to the cooking liquor during the digestion a reducing agent capable of reducing aldehyde groups of cellulose under the conditions prevailing during the digestion.

5. A method as in claim 4, in which said reducing agent is selected from the group consisting of an alkali metal borohydride and hydrogen in combination with a catalyst.

6. A method as in claim 1, which comprises adding during the digestion an oxidizing agent capable of oxidizing aldehyde groups of cellulose under the conditions prevailing during the digestion.

7. A method as in claim 6, in which said oxidizing agent is a polysulfide.

8. In the preparation of cellulose pulp wherein cellulosic materials are digested with an alkaline cooking liquor at an elevated temperature during a cooking period, the improvement which comprises adjusting the pH of the cooking liquor to 11.'513 by adding sulfuric acid thereto in the final stage of the cooking period so that hemicelluloses dissolves in the said liquor during the cook are absorbed on the pulp without substantial degradation of said hemicelluloses or substantial precipitation of lignins.

9. A method as in claim 8 wherein the cooking liquor includes a reducing agent selected from the group consisting of an alkali metal borohydride and hydrogen in combination with a reducing catalyst.

10. A method as in claim 8 wherein the cooking liquor includes a polysulfide.

References Cited UNITED STATES PATENTS 2,041,597 5/1936 Dunbar 16242 3,042,575 7/1962 Hartler 162-80 FOREIGN PATENTS 1,011,120 1948 France.

DONALL H. SYLVESTER, Primary Examiner.

S. LEON BASHORE, Examiner. R. BAIEFSKY, Assistant Examiner. 

1. IN THE PREPARATION OF CELLULOSE PULP WHEREIN CELLULOSIC MATERIALS ARE DIGESTED WITH AN ALKALINE COOKING LIQUOR AT AN ELEVATED TEMPERATURE IN A COOKING PERIOD, THE IMPROVEMENT WHICH COMPRISES DRAWING OFF FROM 50 TO 75% BY VOLUME OF THE COOKING LIQUOR DURING AN EARLY STAGE OF THE COOKING PERIOD WHEN THE PROPORTION OF HEMICELLULOSES THEREIN IS IN THE RANGE 0.1 TO 6% BASED UPON THE CELLULOSIC MATERIAL BEING DIGESTED, AND ADMIXING SAID WITHDRAWN LIQUOR WITH CELLULOSE PULP AND COOKING LIQUOR DURING THE FINAL STAGE OF THE COOKING PERIOD WHEREBY HEMICELLULOSES IN SAID WITHDRAWN LIQUOR ARE ABSORBED ON THE PULP WITHOUT SUBSTANTIAL DEGRADATION OF SAID HEMICELLULOSES.
 8. IN THE PREPARATION OF CELLULOSE PULP WHEREIN CELLULOSIC MATERIALS ARE DIGESTED WITH AN ALKALINE COOKING LIQUOR AT AN ELEVATED TEMPERATURE DURING A COOKING PERIOD, THE IMPROVEMENT WHICH COMPRISES ADJUSTING THE PH OF THE COOKING LIQUOR TO 11.5-13 BY ADDING SULFURIC ACID 